Ion implantation techniques are commonly used as one of the processes employed in the manufacture of integrated circuits, to modify the electrical transport properties in predefined regions of a semiconductor material by doping these regions with a predetermined concentration of impurity atoms. The technique generally involves generating a beam of a preselected specie of ions and directing the beam towards a target substrate. The depth of the ion implant depends, inter alia, on the energy of the ion beam at the substrate. As the density of devices on a single wafer increases and the lateral dimensions of individual devices decrease for ultra-large scale integrated circuits (ULSI), the ability of an ion implanter to form shallow junctions using low energy ions, e.g. of about 0.2 keV to 10 keV, becomes increasingly useful. At the same time, in commercial ion implantation, it is also often useful to be able to process an individual wafer in as short a time as possible. In many applications reduced processing time is achieved by providing a large ion beam current. However, it is often difficult to transport a large current ion beam at low energy due to space charge effects.
U.S. Pat. No. 5,932,882 describes one prior technique in which an ion beam is transported at high energy and then decelerated to a lower energy just before the beam impacts the substrate. The ion implanter of this reference comprises an ion beam generator which includes a source of ions and an extractor electrode assembly for extracting the ions from the source and forming a beam of ions. The extraction electrode assembly comprises one or more electrodes which typically have apertures through which the ion beam is shaped. A magnet adjacent the ion beam generator spatially resolves the beam ions according to their mass. A flight tube transports the beam at a transport energy, and a substrate holder holds a substrate to be implanted with beam ions. A deceleration potential generator connected to apply a deceleration potential to a deceleration lens assembly between the flight tube and the substrate holder decelerates beam ions to a desired implant energy. The deceleration lens assembly located between the flight tube and the substrate holder comprises a plurality of electrodes which typically have apertures through which the ion beam passes.